1. Field of the Invention:
The present invention relates to electronic test fixtures in general and in particular to a fixture for testing different microwave circuitry.
2. Description of The Prior Art:
Monolithic microwave circuits are the subject of much current interest. Coaxial cable is used for signal transmission to and from the circuit, but the circuit itself is generally carried on a planar substrate. Therefore, any test fixture, which holds the substrate while it is being tested, must accommodate these mechanical constraints.
The part of the fixture that interfaces input/output electrical signals between external coaxial transmission line components and the planar type component (e.g. coplanar waveguide or microstrip transmission line) on a carrier substrate within the fixture is known as a coaxial launcher. The substrate contains standard etched gold conductor patterns. The transmission line conductor on the substrate is usually very narrow, from 0.025" to 0.010" (0.6 mm to 0.25 mm). The center contact of the launcher, which also must be very narrow and is usually the same diameter, makes contact with the transmission line conductor of the substrate. Side contacts of the launcher, which are insulated or otherwise spaced from the center contact, make electrical ground contact with the substrate at regions spaced from the central transmission line conductor of the substrate. These outside or grounding contacts must make secure electrical contact, but if the substrate is not perfectly flat, proper electrical contact on both sides of the central region of the substrate is not made.
It is most important that each time the substrate is placed in the fixture the center contact of the launcher only touches the transmission line conductor of the substrate and the outside contact only touch the substrate at locations spaced from the central transmission line conductor. The gap between the transmission line conductor and the spaced grounding regions is no greater than 0.01" (0.25 mm).
In prior art fixtures, the central conductive portion of the launcher, which is of a small diameter, can be broken off relatively easily. Launchers, which are typically gold plated brass and must be machined to relatively close tolerances, are expensive to replace. Also, prior art fixtures are complicated in construction, and it requires much labor and cost to replace the launcher.
Prior art fixtures require the substrate to be mounted within a closed cavity. This prevents access to the device during testing and makes it difficult to see the device or adjust active or passive circuit elements. Moreover, testing of packaged devices is limited by the types of packaging available. Thus, different fixtures have to be made to accommodate different packaged devices.